autoconf 2.71 issues in CI

The base image upgrade to Debian 12 brought two issues. Likely because autoconf was upgraded to 2.71 from v2.69 in Debian 11.

In v9.16, the docs job fails, likely because the autoreconf job is full of warnings.

In v9.18, the Oracle Linux 7 build job fails to find stdatomic.h:

In file included from app.c:28:0:
./include/isc/atomic.h:17:23: fatal error: stdatomic.h: No such file or directory
 #include <stdatomic.h>
                       ^

I think this is autoconf again, not the 3893-make-debian-12-bookworm-base-image-9.18 branch itself, because locally, I don't see the problem on the Oracle Linux 7 image when I pull autoreconf artifact from Debian 11.

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